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Desvignes, Grégory. "L'observation des pulsars au Radiotélescope de Nançay : applications à la recherche de nouveaux objets, à l'étude des systèmes binaires relativistes et à la détection d'un fond d'ondes gravitationnelles." Phd thesis, Université d'Orléans, 2009. http://tel.archives-ouvertes.fr/tel-00496806.
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Un pulsar est une étoile à neutrons en rotation rapide et dotée d'un fort champ magnétique qui peut se manifester en émettant sur tout le spectre électromagnétique. Dans cette thèse, je m'intéresse au rayonnement radio produit par l'étoile. Je commence ainsi par décrire l'instrumentation de dédispersion cohérente à base de GPUs installée au Radiotélescope de Nançay avec deux autres modes d'observation que j'ai développés : un mode pour la recherche de nouveaux pulsar et un mode spectromètre. Une autre partie de ce travail détaille le retraitement en cours du sondage Foster fait à la fin des années 90 à Nançay ainsi que de nouvelles observations ciblées, sur des candidats HESS notamment. Je présente ensuite les résultats obtenus sur les pulsars relativistes J0737-3039A et J1906+0746 avec respectivement des tests de la Relativité Générale et la mesure de la précession géodétique. Des données de polarimétrie ont ainsi permis de déterminer la géométrie du système de PSR J1906+0746. Enfin, je termine par l'analyse des temps d'arrivées de 20 pulsars millisecondes observés à Nançay dans le cadre de l'EPTA, une collaboration européenne pour un réseau de chronométrage pulsar avec pour objectif la détection d'un fond d'ondes gravitationnelles, possible d'ici 5 à 10 ans.
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Livingstone, Margaret Anne. "Timing young pulsars: challenges to standard pulsar spin-down models." Thesis, McGill University, 2010. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=94909.
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Pulsars are rapidly rotating neutron stars which are often noted for their very regular rotation rates. Young pulsars however, frequently exhibit two types of deviations from steady spin down, ``glitches'' - sudden jumps in spin frequency, which provide insight into pulsar interiors, and ``timing noise,'' a smooth stochastic wander of the pulse phase over long time periods. The youngest pulsars also offer a window into the physics that govern pulsar spin down via the measurement of the "braking index'' - a parameter that relates the observable spin frequency of the pulsar with the slowing down torque acting on the neutron star. This thesis discusses long-term timing observations of two young pulsars. First, we present observations of PSR J0205+6449, acquired with the Green Bank Telescope, the Jodrell Bank Observatory and the Rossi X-ray Timing Explorer. We present phase-coherent timing analyses showing timing noise and two spin-up glitches. We also present an X-ray pulse profile analysis showing that the pulsar is detected up to approximately 40 keV and does not vary appreciably over four years. We report the phase offset between the radio and X-ray pulse, showing that the radio pulse leads by 0.10+/-0.01 in phase. We compile measurements of phase offsets for this and other X-ray and gamma-ray pulsars and show that there is no relationship between pulse period and phase offset. Next, we present 10 years of monitoring of PSR J1846-0258 with the Rossi X-ray Timing Explorer. We report the first measurement of the braking index for this pulsar, n=2.65+/-0.01, only the sixth such measurement ever made, and show that the pulsar experienced a small glitch in 2001. In May 2006, PSR J1846-0258 was briefly transformed: it exhibited a series of X-ray bursts, a dramatic increase in the source flux, and significant softening of its X-ray spectrum - behaviours best explained in the context of the magnetar model. PSR J1846-0258 was thus identified as the first rotation-poLes pulsars, des étoiles à neutrons tournant rapidement sur elles-mêmes, sont reconnus pour leur vitesse de rotation très régulière. Les jeunes pulsars, par contre, présentent fréquemment des comportements qui dévient du ralentissement uniforme de leur vitesse de rotation: des glitchs, variations brutales de la fréquence de révolution qui nous aident à comprendre l'intérieur des plusars, et le bruit chronométrique, une variation stochastique de la phase de rotation sur une longue échelle de temps. Les pulsars les plus jeunes nous offrent aussi un aperçu de la physique qui gouverne le ralentissement de la vitesse de rotation par l'indice de freinage, un paramètre qui relie la fréquence de rotation d'un pulsar au torque qui agit sur lui, et dont la valeur diminue graduellement. Cette thèse discute du chronométrage à long terme de deux jeunes pulsars. Tout d'abord, nous présentons des observations de PSR J0205+6449 acquises avec l'Observatoire de Green Bank, l'Observatoire Jodrell Bank ainsi que le Rossi X-ray Timing Explorer. Nous présentons l'analyse du chronométrage à phase cohérente montrant du bruit chronométrique ainsi que deux glitchs. Nous présentons aussi une analyse du profil du pulse en rayons X montrant que le pulsar est détectable jusqu'à ~40 keV et ne varie pas significativement sur quatre ans. Nous rapportons une mesure de la différence de phase entre le pulse radio et le pulse en rayons X, montrant que le pulse radio précède le pulse en rayons X par 0.10+/-0.01. Une compilation des différences de phase pour ce pulsar et d'autres qui émettent en rayons X et en rayons gamma montre qu'il n'y a aucune relation entre la période de rotation et la différence de phase. Ensuite, nous présentons 10 années de suivi de PSR J1846-0258 avec le Rossi X-ray Timing Explorer. Nous rapportons la première mesure de l'indice de freinage pour ce pulsar, n=2.65+/-0.01, le sixième indice mesuré à ce jour, et montrons que ce pul
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Jacoby, Bryan Anthony Hillenbrand Lynne A. "Recycled pulsars /." Diss., Pasadena, Calif. : California Institute of Technology, 2005. http://resolver.caltech.edu/CaltechETD:etd-01272005-015012.
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Brook, Paul Richard. "The variability of radio pulsars." Thesis, University of Oxford, 2015. http://ora.ox.ac.uk/objects/uuid:65ae413c-cd12-408b-843c-60886cecf1b7.
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Neutron stars are amongst the most exotic objects known in the universe; more than a solar mass of material is squeezed into an object the size of a city, leading to a density comparable to that of an atomic nucleus. They have a surface magnetic field which is typically around a trillion times stronger than the magnetic field here on Earth, and we have observed them to spin up to around 700 times per second. The existence of neutron stars was first proposed by Baade and Zwicky in 1934 but later graduated from theory to fact in 1967 as the first pulses were detected by Jocelyn Bell-Burnell, a then graduate student at the University of Cambridge. There are now well over 2000 neutron stars whose radio emission beams point at, and have been detected on Earth. We call these objects pulsars. Because of their remarkable properties, pulsars are very useful to physicists, who can employ them as precision timing tools due to the unwavering nature of their emission and of their rotation. Having an array of ultra-accurate clocks scattered throughout our galaxy is very useful for performing astrophysical experiments. In particular, precise pulsar timing measurements and the models that explain them, will permit the direct detection of gravitational radiation; a stochastic background initially, and potentially the individual signals from supermassive black hole binaries. Our models of pulsar behaviour are so precise that we are now able to notice even slight departures from them; we are starting to see that unmodelled variability in pulsars occurs over a broad range of timescales, both in emission and in rotation. Any unmodelled variability is, of course, detrimental to the pulsar's utility as a precision timing tool, and presents a problem when looking for the faint effects of a passing gravitational wave. We are hoping that pulsar timing arrays will detect gravitational radiation in the coming decade, but this depends, in part, on our ability to understand and mitigate the effects of the unmodelled intrinsic instabilities that we are observing. One important clue as to the nature of the variability in pulsar emission and rotation, is the emerging relationship between the two; we sometimes observe correlation on timescales of months and years. We have been observing pulsars for almost fifty years and our expanding datasets now document decades of pulsar behaviour. This gives us the ability to investigate pulsar variability on a range of timescales and to gain an insight into the physical processes that govern these enigmatic objects. In this thesis I describe new techniques to detect and analyse the emission and rotational variability of radio pulsars. We have employed these techniques on a 24 year pulsar dataset to unearth a striking new example of a dramatic and simultaneous shift in a pulsar's emission and rotation. We hypothesise that this event was caused by an asteroid interaction, although other explanations are also possible. Our variability techniques have also been used to analyse data from 168 young, energetic pulsars. In this thesis we present results from the nine most interesting. Of these, we have found some level of correlated variability in seven, one of which displays it very strongly. We have also assessed the emission stability of the NANOGrav millisecond pulsars and have found differing degrees of variability, due to both instrumental and astrophysical causes. Finally, we propose a method of probing the relationship between emission and rotation on short-timescales and, using a simulation, we have shown the conditions under which this is possible. Throughout the work, we address the variability in pulsar emission, rotation and links between the two, with the aim of improving pulsar timing, attaining a consolidated understanding of the diverse variable phenomena observed and elucidating the evolutionary path taken by pulsars.
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Yardley, Daniel Roger Billing. "Studying gravitational waves with pulsars : results from the Parkes pulsar timing array." Thesis, The University of Sydney, 2011. https://hdl.handle.net/2123/29163.
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This thesis addresses the problem of gravitational-wave (GW) detection using radio timing observations of pulsars. We study GW signals in real and simulated pulsar timing observations, and describe the astrophysical implications for cases in which no GW signal is detected. We simulate timing observations from a range of hypothetical pulsar timing array projects. The pulse arrival times are then perturbed by an individual source of GWs. One of the simulated data sets comprises an array of 20 pulsars timed with a root-mean-square residual of 10 ns over 10 years. If there is no detectable GW signal in this data set, then the merger rate of supermassive black-hole binaries (SMBHBs) with a chirp mass of 109 solar masses is less than one merger every 105 years up to a redshift of z = 2. This constraint rules out estimates of the SMBHB merger rate based on hierarchical galaxy formation with standard assumptions on the merger parameters. Applying a similar analysis to recently published observations from the Parkes Pulsar Timing Array (PPTA) yields a constraint on the merger rate of SMBHBS of less than one merger every five years for SMBHBs with chirp mass ~ 1010 solar masses up to a redshift of z = 2. The results also indicate that it is unlikely that an individual GW source could be detected with existing data sets.
We consider the signal caused by an isotropic stochastic gravitational-wave background (GWB), and show that, with a few more years of observations, either the GWB will be detected or the parameter space of most current galaxy evolution models will be significantly constrained. An analysis of the cross-correlation between the timing residuals of different pulsars in the PPTA shows that there is no detectable GWB signal in the current data. The GWB detection statistic is dominated by only a few pulsars in the current PPTA data. There are good prospects for detection of the GWB using radio timing of pulsars in the next decade. We conclude that the effect of instabilities in realisations of the terrestrial timescale and inaccuracies in the solar system ephemeris must be removed from the timing residuals in order to detect a GW signal in pulsar timing observations in the future. The forthcoming International Pulsar Timing Array project will also significantly increase the probability of detection of GW signals using pulsar timing.
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Octau, Franck. "Exploration d'un grand relevé à Nançay et diversité de la population de pulsars." Thesis, Orléans, 2017. http://www.theses.fr/2017ORLE2042/document.
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Depuis la découverte du premier pulsar en 1967, nous connaissons désormais plus de 2500 pulsars aujourd’hui. Les pulsars offrent un champ d’études considérable : depuis l’étude des propriétés du milieu interstellaire et l’étude de la magnétosphère des pulsars jusqu’aux tests de la gravité en champ fort et la caractérisation d’un fond d’ondes gravitationnelles d’origine cosmologique. Cela explique pourquoi nous continuons de chercher de nouveaux pulsars de nos jours. Après des découvertes de pulsars millisecondes dans les sources non identifiées du Fermi Large Area Telescope, un programme de recherche de nouveaux pulsars a été mené à partir de 2012 par G. Desvignes. Observant à 1.4 GHz avec une haute résolution temporelle et fréquentielle, le programme SPAN512 a été conçu pour la recherche de pulsars rapides et lointains situés dans le plan Galactique. Nous décrirons les méthodes d’analyse mises en place pour traiter les données afin de trouver de nouveaux pulsars, méthodes soit basées sur la stabilité de la période de rotation des pulsars soit sur leur émission d’impulsions individuelles. Nous présenterons aussi l’état actuel de l’analyse du programme SPAN512 et les découvertes effectuées, plus particulièrement du pulsar trouvé au cours de ce travail de thèse, PSR J2055+3829, un pulsar milliseconde de période de rotation de 2.08 ms appartenant à un système de type « Veuve Noire ». Ce sera l’occasion de présenter les études chronométriques réalisées pour trouver l’éphéméride de ce pulsar et, dans le même temps, j’en profiterai pour parler d’une analyse similaire faite sur le pulsar J1618-3921, un pulsar dans une orbite excentrique. Enfin, nous présenterons des études polarimétriques de pulsars réalisées à la lumière d’un nouveau modèle, le modèle du vecteur tournant décentré (DRVM). Nous montrerons qu’un champ magnétique hautement décentré peut expliquer les variations brusques de l’angle de polarisationSince the discovery of the first pulsar in 1967, we know over 2500 pulsars today. Pulsars offer a broad range of studies: from the study of the properties of interstellar medium and of pulsar magnetospheres up to test of gravity in the strong-field regime and the characterisation of the cosmological Gravitation Waves background. This explains why we keep searching pulsars nowadays. After successful detections of new millisecond pulsars in Fermi Large Area Telescope unassociated sources at Nançay, a blind pulsar survey was initiated in 2012 by G. Desvignes. Conducted at 1.4 GHz with short sampling time and narrow frequency channels, the SPAN512 was designed to find fast and distant pulsars within the Galactic plane. We describe the methods to analyse data in order to find new pulsars, thanks to their spin stability or tto their single pulses. We will also describe the current status of the survey and the discoveries, more especially the pulsar discovered during this thesis, PSR J2055+3829, a 2.08 ms pulsar in a black widow system. It will be the opportunity to present the radio timing analysis of this pulsar and, in the same time, we will describe similar studies conducted on the pulsar J1618-3921, a pulsar in an eccentric orbit. Finally, we present some polarisation studies of pulsars in light of a new model, the Decentred Rotating Vector Model (DRVM). We will show that a highly decentred dipole may explain abrupt variations of polarisation profiles
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Edwards, Russell T. "Pulsar searching /." Australian Digital Theses Program, 2001. http://adt.lib.swin.edu.au/public/adt-VSWT20050323.141044.
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Thesis (Ph.d.) - Swinburne University of Technology, 2001.Submitted for the degree of Doctor of Philosophy, Swinburne University of Technology, 2001. Typescript. Includes bibliographical references (p. 122-131).
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Icdem, Burcin. "Viscous Time Scale In Accreting Powered Pulsars And Anomalous X-ray Pulsars." Master's thesis, METU, 2011. http://etd.lib.metu.edu.tr/upload/12613373/index.pdf.
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In this thesis we analyse X-ray data of accretion powered low mass and high mass X-ray binaries to understand the nature of their accretion mechanisms by searching for some clues of viscous time-scales of their accretion discs, if they have, in their low frequency power density spectra created from their long-term X-ray observations, or by doing pulse timing analysis with much shorter X-ray data to detect the effects of torque fluctuations caused by the accreting material on the pulsar.
The low mass and high mass X-ray binaries we analysed have breaks in their power density spectra, which are attributed to the role of viscosity in the formation of accretion discs. Although, the time-scales corresponding to these break frequencies are smaller than the predictions of the Standard theory of accretion discs, the sources give consistent results among themselves by displaying the expected correlation between their break and orbital frequencies. The correlation curve of LMXBs implies thicker appearing accretion discs than those assumed by the theory. The dichotomy
of the HMXBs on this curve points out the different origins of accretion that these sources may have, and offers a way to distinguish the stellar-wind fed systems from the Roche-lobe overflow systems.
The timing and spectral analysis of Swift J1626.6-5156 reveal a correlation between the spin-up rate and the luminosity of the source implying that the pulsar is accretion-powered. This correlation together with the characteristics of the X-ray spectra enables us to estimate the magnetic field and the distance of the source.
The AXP 1E 2259+586 does not display any signs of viscous time-scale in its low frequency power density spectra, and its pulse timing analysis gives a much smaller torque noise value than that expected from accretion powered pulsars. In addition, the analysis results presented in this thesis reveal magnetar-like glitches which differ than those of radio pulsars, due to the presence of the strong magnetic field of the pulsar. These results eliminate the possibility that the AXP is an accretion-powered pulsar.
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Voûte, Jan Lodewijk Lancelot. "The many shapes of giant pulses radio pulsar research at WSRT /." [S.l. : Amsterdam : s.n.] ; Universiteit van Amsterdam [Host], 2001. http://dare.uva.nl/document/92733.
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Tang, Pui-shan Anisia, and 鄧佩姗. "Thermal X-ray pulses resulting from pulsar glitches." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1999. http://hub.hku.hk/bib/B29790992.
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Tang, Pui-shan Anisia. "Thermal X-ray pulses resulting from pulsar glitches /." Hong Kong : University of Hong Kong, 1999. http://sunzi.lib.hku.hk/hkuto/record.jsp?B2089675X.
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Herfindal, Jeffrey. "Nulling Periodicities in Pulsars." ScholarWorks @ UVM, 2008. http://scholarworks.uvm.edu/graddis/108.
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Numerous studies of conal pulsars, e.g. B1133+16, have revealed fluctuation features and a steady null fraction. Sensitive Arecibo observations provide an unprecedented ability to detect nulls and confirm previously found fluctuation features. By replacing each pulse with a scaled version of the average profile, we were able to quench all subpulse modulation, dubbed pulse-modulation quelling (PMQ). It was surprising to note that the low-frequency feature observed in the natural longitude resolved fluctuation spectra (LRF) persisted in the PMQ LRFs! It appears that we can conclude, then, that the nulls themselves reflect whatever underlying periodicity is responsible for the low-frequency feature. Conversely, the aggregate fluctuation power of the low frequency feature changes little whether the pulse modulation is quelled or not, implying that the feature fluctuations are produced by the nulls! These conclusions are perplexing because (with very unusual exceptions) no obvious or regular periodicities have heretofore been attributed to null occurrence.
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Hassall, Tom. "Observing pulsars with LOFAR." Thesis, University of Manchester, 2012. https://www.research.manchester.ac.uk/portal/en/theses/observing-pulsars-with-lofar(1860a120-1c7a-4d64-a281-f482f3f53614).html.
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The LOw Frequency ARray (LOFAR) is the first of a new generation of telescopes which, instead of using a single large dish, will combine the signals from thousands of antennas. Combining these signals creates a virtual beam, which can be 'pointed' like traditional (dish-like) telescopes, but with significantly improved angular resolution and sensitivity at low frequencies. This unique configuration also allows LOFAR to observe in several modes which are not possible with other telescopes, including forming multiple beams, and simultaneously taking imaging and beam-formed data. In this thesis we present some of the early science and commissioning work which has been undertaken with LOFAR. Wide-band simultaneous observations of four pulsars using LOFAR, the Lovell Telescope and the Effelsberg 100-m Telescope are used to constrain emission heights and examine the evolution of the average pulse pulse profile between 50~MHz and 8~GHz. We find that the radio emission from all of the pulsars observed comes from a region not larger than 400~km in size, and also see pulse profile evolution that deviates significantly from what is expected from radius-to-frequency mapping. The same observations are also used to probe the interstellar medium (ISM). The ISM imparts a frequency-dependent dispersive delay on pulses propagating through it. This delay is well understood, and well measured but there are many second-order effects of the ISM, like refraction and anomalous dispersion, which have not yet been detected, and which are expected to scale steeply with frequency. These effects are potentially only observable at low frequencies, and if measured, can be used to determine the distribution of electrons and other charged particles along the line-of-sight. We find no evidence of these effects, instead finding that that the cold dispersion law is accurate to better than 1 part in 100,000 between ~40 and ~180 MHz. The absence of any of these effects in our data was used to place upper limits on some of the properties of the ISM. From these upper limits, we also show that delays from the ISM are <50 ns at normal pulsar timing frequencies (1400 MHz).We also present an investigation in to the behaviour of the single pulses of PSR B0809+74 at low-frequencies. We show that the drift rate of PSR B0809+74 is extremely constant on timescales of a few hours and a few years and compare the shape of driftbands at low frequencies to their high frequency counterparts. We identify a slowly evolving microstructure, which is persistent for several pulse periods, allowing us to solve the aliasing problem and determine the speed of the carousel. We also find evidence for 'partial nulls', in which one subpulse is extinguished without interrupting the emission of the others.
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Sanwal, Divas. "Optical study of pulsars /." Digital version accessible at:, 1999. http://wwwlib.umi.com/cr/utexas/main.
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Cameron, Andrew [Verfasser]. "Innovative Pulsar Searching Techniques : or Fantastic Pulsars and How to Find Them / Andrew Cameron." Bonn : Universitäts- und Landesbibliothek Bonn, 2018. http://d-nb.info/1162953063/34.
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Larroche, Olivier. "Mécanismes de rayonnement des pulsars." Phd thesis, Université Paris Sud - Paris XI, 1987. http://tel.archives-ouvertes.fr/tel-00785857.
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On étudie, du point de vue théorique ainsi que par des simulations numériques, l'instabilité vis-à-vis du rayonnement de courbure d'un faisceau de particules chargées guidées par un très fort champ magnétique courbé, qui est intéressante en tant que mécanisme de rayonnement radio des pulsars. Les conditions de croissance sont un gradient de densité assez raide sur la frontière extérieure du faisceau et des fréquences élevées, satisfaisant une condition non-WKB.
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Breton, René. "Radio pulsars in binary systems." Thesis, McGill University, 2009. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=40755.
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This thesis focuses on the study of binary radio pulsars, their evolution and some specific use of their properties to investigate fundamental physics such as general relativity and other gravitational theories. The work that we present here is organized in three main parts. First, we report on the study of PSR J1744-3922, a binary pulsar presenting a peculiar `flickering' flux behavior as well as spin and orbital properties that do not correspond to the expectations of standard evolution scenarios. We investigated the nature of this flux behavior. We also studied the pulsar's properties in relationship to the binary radio pulsar population and proposed the existence of an as yet unidentified class of binary pulsars. Second, we conducted an in-depth analysis of the eclipses in the relativistic double pulsar system PSR J0737-3039A/B. During these eclipses, the `A' pulsar partly disappears for ~30 s behind its companion, `B'. The eclipse light curve displays a complex structure of flux modulation that is synchronized with the rotation of pulsar B. We worked on improving our understanding of the eclipse phenomenology and more particularly the modulation phenomenon. From our modeling of the eclipses, we precisely determined the geometry of pulsar B in space and used this information to study the temporal behavior of the eclipses, which revealed that pulsar B precesses around the angular momentum of the system in a way that is consistent with the prediction of general relativity. Third, we searched for the signature of latitudinal aberration in the pulse profile of pulsar A in PSR J0737-3039A/B. This relativistic effect should cause a periodic variation in the separation between the two pulse components of pulsar A on an orbital time scale. The non-detection of this effect allows us to put an upper limit on its amplitude, which constrains the geometry of pulsar A with respect to our line of sight as well as its emission geometry.Cette thèse se concentre sur l'étude des pulsars en systèmes binaires, leur évolution, ainsi que certains usages de leurs propriétés pour comprendre la physique fondamentale dont la relativité générale et les théories gravitationnelles alternatives. Le travail de cette thèse comprend trois parties principales.En premier lieu, nous présentons l'étude de PSR J1744-3922, un pulsar binaire démontrant d'étranges fluctuations d'intensité lumineuse ainsi que des propriétés orbitales et de rotation qui ne correspondent pas aux prédictions des scénarios évolutifs conventionnels. Nous analysons d'abord les fluctuations d'intensité lumineuse. Nous étudions ensuite la nature de ce pulsar en relation avec la population de pulsars radio en systèmes binaires et proposons l'existence d'une classe de pulsars binaires qui n'avait pas encore été mise à jour.Deuxièmement, nous avons réalisé une analyse en profondeur des éclipses du pulsar double relativiste PSR J0737-3039A/B. Durant ces éclipses, le pulsar `A' disparaît partiellement pendant une trentaine de secondes derrière son compagnon, `B'. La courbe de lumière des éclipses montre une complexe structure de modulation d'intensité qui est synchronisée avec la rotation du pulsar B. Les travaux présentés ici ont pour but de mieux comprendre la phénoménologie des éclipses et visent plus particulièrement le phénomène de modulation. La modélisation des éclipses nous a permis de précisément déterminer la géométrie du pulsar B dans l'espace et d'en déduire son évolution temporelle. Nous concluons que le pulsar B subit une précession de son axe de rotation autour du moment angulaire du système selon un taux et une direction en accord avec la prédiction de la relativité générale.Pour conclure, nous avons recherché la présence d'aberration latitudinale dans le profil du pulse du pulsar A, toujours dans le double pulsar PSR J0737-3039A/B. Cet effet relativiste devrait causer une
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Brazler, Karen Tracy Susan. "TeV astronomy of millisecond pulsars." Thesis, Durham University, 1991. http://etheses.dur.ac.uk/5971/.
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This thesis is concerned with the detection of pulsed TeV γ-rays from millisecond pulsars. These stars appear to include some very efficient producers of high energy particles, but the mechanisms by which they produce TeV γ-rays are still a matter of debate. After an introductory section, there is a brief description of the principles used in the atmospheric Cerenkov technique. The design and operation of the University of Durham atmospheric Cerenkov telescopes are reviewed. The main analysis techniques used to search for periodic signals are then described. The effects on periodic signals of binary motion of a source are discussed. These are a particularly important consideration for observations of millisecond pulsars, where high timing accuracy is required. One of the problems of detecting TeV sources is the cosmic ray background. A means of rejecting background events in TeV γ -ray telescopes is considered in chapter 5. The technique is developed for the Durham Mark III telescope. Substantial rejection of the cosmic ray background is achieved, with minimal loss of source events. The evolutionary scenarios which lead to the formation of millisecond pulsars are outlined. Two models for 7-ray emission are discussed briefly and applied to six known millisecond pulsars. Empirical results on these and two other pulsars are also presented. In particular, a detection of PSR 1855+09 is reported, and an upper limit to the flux from PSR 1957+20 is derived. All the empirical fluxes are compatible with the emission models, but the 'polar gap' model may be favoured. The final chapter summarises the results obtained and suggests some directions for future work on the 7-ray emission from millisecond pulsars.
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Smedley, Sarah Louise. "Formation of binary millisecond pulsars." Thesis, University of Cambridge, 2015. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.709132.
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Kiel, Paul Douglas. "Populating the galaxy with pulsars." Swinburne Research Bank, 2009. http://hdl.handle.net/1959.3/64888.
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Thesis (Ph.D) - Swinburne University of Technology, Faculty of Information & Communication Technologies, 2009.A dissertation presented in fulfillment of the requirements for the degree of Doctor of Philosophy, [Faculty of Information and Communication Technologies], Swinburne University of Technology, 2009. Typescript. Bibliography: p. 207-223.
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Larroche, Olivier. "Mécanismes de rayonnement des pulsars." Grenoble 2 : ANRT, 1987. http://catalogue.bnf.fr/ark:/12148/cb37607032p.
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Guillemot, Lucas. "Détections de pulsars milliseconde avec le FERMI Large Area Telescope." Phd thesis, Bordeaux 1, 2009. http://tel.archives-ouvertes.fr/tel-00432706.
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Le satellite Fermi a été lancé le 11 juin 2008, avec à son bord le Large Area Telescope (LAT). Le LAT est un télescope sensible au rayonnement gamma de 20 MeV à plus de 300 GeV. Au début de l'activité de Fermi, neuf pulsars jeunes et énergétiques étaient connus dans le domaine gamma. Le nombre de détections de pulsars par le LAT prédit avant lancement était de plusieurs dizaines au moins. Le LAT permettait également l'étude des pulsars milliseconde (MSPs), jamais détectés avec certitude à très haute énergie jusqu'alors. Cette thèse aborde dans un premier temps la campagne de chronométrie des pulsars émetteurs radio et/ou X, candidats à la détection par le LAT, en collaboration avec les grands radiotélescopes et télescopes X. Cette campagne a permis la recherche de signaux gamma pulsés avec une grande sensibilité. En outre, la plupart des MSPs galactiques ont été suivis dans le cadre de cette campagne, sans biais de sélection a priori sur cette population d'étoiles. Pour la première fois, des pulsations ont été détectées pour huit MSPs galactiques au-dessus de 100 MeV. Quelques bons candidats à une détection prochaine apparaissent. Une recherche similaire a été conduite pour des MSPs d'amas globulaires, sans succès à présent. L'analyse des courbes de lumière et des propriétés spectrales des huit MSPs détectés révèle que leur rayonnement gamma est relativement similaire à celui des pulsars ordinaires, et est vraisemblablement produit dans la magnétosphère externe. Cette découverte suggère que certaines sources non identifiées sont des MSPs, pour l'instant inconnus.
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Wu, Man-ho, and 胡文浩. "High-energy radiation from millisecond pulsars." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2013. http://hdl.handle.net/10722/193484.
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As the successor of the Energetic Gamma Ray Experiment Telescope, the Large Area Telescope on board the Fermi Gamma-ray Space Telescope has measured the pulsations from over a hundred pulsars, a rapidly growing population in the gamma-ray sky. Pulsars spinning as fast as a few to ten milliseconds, the millisecond pulsars (MSPs), constitute over 30% of the population, with most of which found in binary systems. Many MSPs are found to have low-mass companions with masses less than ~ 0:1 of that of the Sun. Observations reveal that in these systems, the companion is irradiated by the radiation and/or the wind from the pulsar. It is believed that isolated MSPs in the Galactic field were once a member of such kind of systems, and have ablated their companion away. The gamma-ray emission from the original Black Widow pulsar was studied using data from the Fermi-LAT. The emission was found to depend on the orbital phase and an extra higher-energy component was observed near the inferior conjunction. The results can be explained by an inverse Compton (IC) process associated with the ultra-relativistic pulsar wind. PSR B1957+20 is the first black widow system from which evidence on interaction of unshocked pulsar wind is observed. In addition, diffuse X-rays were found from the globular cluster (GC) 47 Tucanae. It is believed that the observed gamma-rays and the diffuse X-rays can be produced via inverse Compton process between the background soft photons and the pulsar wind from the MSPs residing in the GC. The observation of diffuse X-rays from 47 Tucanae provides constraints on the energetics and the emission region for the IC scenario. They provide good examples and guides for search strategies of similar emissions in other black widow systems and globular clusters.published_or_final_version
Physics
Master
Master of Philosophy
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Espinoza, Cristobal Manuel. "The rotational history of young pulsars." Thesis, University of Manchester, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.508978.
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Ferdman, Robert Daniel. "Binary pulsars: evolution and fundamental physics." Thesis, University of British Columbia, 2008. http://hdl.handle.net/2429/370.
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In the standard theory of pulsar spin-up, a neutron star (NS) in a binary system accretes matter from its companion star; this serves to transfer angular momentum to the NS, increasing the spin frequency of the pulsar. Measurement of the orbital parameters and system geometry, and in particular the final system masses, thus provide important constraints for theories regarding binary evolution. We present results from an investigation of three binary pulsar systems.
PSR J1802-2124 is in an intermediate-mass pulsar binary system with a massive white dwarf companion in a compact orbit with a period of 16.8 hours. We have per-formed timing analysis on almost five years of data in order to determine the amount of Shapiro delay experienced by the incoming pulsar signal as it traverses the potential well of the companion star on its way to Earth. We find the pulsar in this system to have a relatively low mass at 1.24 ± 0.11 M®, and the companion mass to be 0.79 ± 0.04111.).We argue that the full set of system properties indicates that the system underwent a common-envelope phase in its evolutionary history.
The double pulsar system PSR 0737-3039A/B is a highly relativistic double neutron star (DNS) binary, with a 2.4-hour orbital period. The low mass of the second-formed NS, as well the low system eccentricity and proper motion, have suggested a different evolutionary scenario compared to other known DNS systems. We describe analysis of the pulse profile shape over six years of observations, and present the constraints this provides on the system geometry. We find the recycled pulsar in this system, PSR 0737-3039A,to have a low misalignment angle between its spin and orbital angular momentum axes, with a 95.4% upper limit of 14 °, assuming emission from both magnetic poles. This tight constraint lends credence to the idea that the supernova that formed the second pulsar was relatively symmetric, possibly involving electron captures onto an 0-Ne-Mg core.
We have also conducted timing analysis of PSR J1756-2251 using four years of data, and have obtained tight constraints on the component masses and orbital parameters in this DNS system. We have measured four post-Keplerian timing parameters for this pulsar; the Shapiro delay s parameter, with a 5% measured uncertainty, is consistent at just above the la level with the predictions of general relativity. The pulsar in this system has a fairly typical NS mass of 1.312 ± O.017M®, and the companion NS to be relatively light, with a mass of 1.2581017 Mo. This, together with the somewhat low orbital eccentricity of this system (e 0.18), suggests a similar evolution to that of the double pulsar. We investigate this further, through a similar pulse profile analysis to that performed with PSR J0737-3039A, with the goal of constraining the geometry of this system.
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Kasian, Laura Elizabeth. "Radio observations of two binary pulsars." Thesis, University of British Columbia, 2012. http://hdl.handle.net/2429/41515.
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The study of pulsars in binary systems produces a wide variety of scientific results, including unique tests of general relativity and constraints on the equation of state of matter at extremely high densities. Through pulsar timing (which utilizes the fact that pulsars are precise clocks), it is possible to precisely measure the orbital parameters and masses of some binary pulsars, which can place constraints on their mass transfer histories. We present timing results for two binary pulsars.
The intermediate-mass binary pulsar (IMBP) PSR J0621+1002 is a mildly recycled pulsar in an 8.3-day orbit around a massive white dwarf (WD) companion. It is one of only two known IMBPs with a precise mass measurement. We collected 9 days' worth of data using the 305-metre Arecibo telescope (allowing for full orbital coverage), which we used to improve constraints on the advance of periastron, and in turn, the pulsar and companion masses (to 1.53^{+0.10}_{-0.20}M☉ and 0.76^{+0.28}_{-0.07} solar masses, respectively) and inclination angle of the system. These results, combined with the relatively long orbital period, suggest a disk accretion evolutionary scenario involving short-lived periods of hyper-accretion, in which a moderate amount of mass has been transferred to the neutron star (NS).
PSR J1906+0746 is a young pulsar in a compact ~4-hour orbit around a companion that was discovered in the early stages of the PALFA survey using the 305-metre Arecibo telescope. We present the timing results using data collected between 2005 to 2009 using the Green Bank, Arecibo, and Nançay telescopes. We have measured the advance of periastron, the time dilation and gravitational redshift parameter, and the orbital decay, and we find the pulsar and companion masses to be 1.323^{+0.011}_{-0.011}M☉ and 1.290^{+0.011}_{-0.011} solar masses, respectively. Although the companion may be a second NS, it is more likely to be a massive WD. The system's evolution probably involved a substantial transfer of mass from the WD progenitor onto the NS progenitor through Roche-lobe overflow accretion, followed by the formation of the WD, and a short common envelope phase, and finally the ejection of the envelope and the pulsar-forming supernova.
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Nespoli, Elisa. "Multiwavelength study of accretion-powered pulsars." Doctoral thesis, Universitat de València, 2010. http://hdl.handle.net/10803/41717.
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My PhD thesis consists in a multi-frequency approach to High Mass X-ray Binaries (HMXBs), using infrared and X-ray data.
On one side, my research project aimed at the identification and characterization of IR counterparts to obscured HMXBs in the Scutum and Norma inner galactic arms. The identification of optical/IR counterparts to HMXBs is a necessary step to undertake detailed studies of these systems. With data limited to the high-energy range, the understanding of their complex structure and dynamics cannot be complete.
In the last years, INTEGRAL has revealed the presence of an important population of heavily absorbed HMXBs in the Scutum and Norma regions, virtually unobservable below 4 keV. Optical counterparts to these obscured sources are hardly observable, due to the high interstellar extinction.
I selected candidate counterparts by means of a photometric search for emission-line stars in the error boxes of the X-ray sources detected by INTEGRAL. With this objective, I built up (Brγ-K)-(H-K) and (HeI-K)-(H-K) IR color-color diagrams, in which emission-line stars are expected to show up below the absorption-line stars sequence. I applied this technique to search for counterparts to Be/XRBs, whose transient nature prevents the counterpart identification with follow-up X-ray observations with high spatial resolution. For each field, one to four candidate counterparts were identified.
I also took spectra of proposed counterparts. The confirmation and spectral classifications of the systems led to unveil the nature of nine INTEGRAL objects.
On the other hand, the work intended to provide for the first time a systematic study of four Be/XRBs during giant (type II) outbursts. I employed RXTE data, applying the three techniques of color-color/hardness-intensity diagrams (CD/HID), spectral fitting and Fourier power-spectral analysis, simultaneously, and using the retrieved results and correlations to try to define and characterize spectral states for this class of systems. In this way I followed both a model-independent (CD/HID) and model-dependent approach (spectral fits) to investigate the rapid aperiodic variability as a function of spectral sates in HMXBs.
I obtained lightcurves, energy and power spectra for a total number of 320 observations. From X-ray colors, spectral and timing fitting, I clearly identified in all the four systems two different spectral states, i.e. the Diagonal Branch (DB) and Horizontal Branch (HB).
The HB corresponds to a lower-flux state, with larger rms than the DB. Also, the power-law photon index decreases with flux in the HB, while stays constant or increases in the DB. The HB shows lower characteristic frequencies of the noise components than in the DB. The cyclotron resonant scattering features are generally associated with the DB, while absent or weaker during the DB. We showed how the transition between the two states may correspond to the transition from the standing shock emission to the thermal mound emission due to the turning point from super-Eddington luminosity regime (DB) to sub-Eddington luminosity regime (HB).
From color, spectral and timing point of view, differences among systems easily distinguishes two subgroups, with the slower pulsars, KS 1947+300 and EXO2030+375, on one side, and the faster ones, 4U 0115+63 and V 0332+53, on the other. The first group is characterized by softer spectra in the HB compared with the other systems. Hysteresis is not observed in the slower pulsars, while it is evident in V 0332+53 and 4U 0115+63. Cyclotron resonant scattering features are crucial in the spectral shape of V 0332+53 and 4U 0115+63, where also a harmonic is observed in the 3-30 keV energy range. They are instead absent or very weak in the first group. According to timing features, a strong difference between the two groups is the presence of QPOs in the faster pulsars.El trabajo de tesis consiste en un estudio multifrecuencia de Binarias de rayos X de Alta Masa (HMXBs), utilizando datos infrarrojos y en banda X. E n l o s úl t i m o s a ñ o s l a m i s i ó n I N T E G R A L d e l a A g e n c i a E s p a c i a l E u r o p e a h a r e v e l a d o l a p r e s e n c i a d e u n a i m p o r t a n t e p o b l a c i ó n d e H M X Bs e n l o s b r a z o s g a l ác t i c o s d e S c u t u m y N o r m a . E n e f e c t o , l a m a y o r í a d e l a s n u e v a s H M X Bs d e s c u b i e r t a s p o r INTEGRAL e s t án f u e r t e m e n t e a b s o r b i d a s , v i r t u a l m e n t e i n o b s e r v a b l e s a e n e r g ía s i n f e r i o r e s a l o s 4 k e V . E s o e x p l i c a c o m o n o f u e r o n d e t e c t a d a s p o r l a s m i s i o n e s d e r a y o s X a n t e r i o r e s a I N T E G R A L . L a s c o n t r a p a r t i d a s ó p t i c a s d e e s t a s f u e n t e s o s c u r e c i d a s s o n d i f í c i l m e n t e o b s e r v a b l e s a c a u s a d e l a e l e v a d a e x t i n c i ó n i n t e r e s t e l a r. Por un lado, el objetivo de este proyecto fue la i d e n t i f i c a c i ó n d e c o n t r a p a r t i d a s i n f r a r r o j a s a l a s f u e n t e s d e r a y o s g a m m a d e s c u b i e r t a s p o r I N T E G R A L . La identificación se llevó a cabo a partir de fotometría en las bandas H, K, Br y HeI, y espectroscopía de resolución intermedia en la banda K. Para cada campo de INTEGRAL analizado se identificaron de una a cuatro posibles contrapartidas; nueve objetos desconocidos fueron clasificados como HMXBs. Por otro lado, se llevó a cabo el primer estudio sistemático de cuatro HMXBs durante outbursts gigantes, con el objetivo de definir estados espectrales en esta clase de binarias de rayos X. Utilizando datos del satélite RXTE, se extrajeron curvas de luz, espectros de energía y espectros de potencia de 320 observaciones. A través de los tres diferentes tipos de análisis (colores en banda X, ajustes espectrales y temporales), se identificaron en todas las fuentes dos estados espectrales, denominados “Rama Horizontal” (HB) y “Rama Diagonal” (DB). Al mismo tiempo, las diferencias entre las fuentes analizadas permitieron definir dos subgrupos de sistemas, con características espectrales y temporales propias: por una lado los púlsares lentos, y por otro los más rápidos.
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Nieder, Lars [Verfasser]. "Binary Gamma-ray Pulsars / Lars Nieder." Hannover : Gottfried Wilhelm Leibniz Universität, 2020. http://d-nb.info/1229615091/34.
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Lyon, Robert James. "Why are pulsars hard to find?" Thesis, University of Manchester, 2016. https://www.research.manchester.ac.uk/portal/en/theses/why-are-pulsars-hard-to-find(f15226ec-355d-4794-b2b8-e0a8e793948e).html.
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Searches for pulsars during the past fifty years, have been characterised by two problems making their discovery difficult: i) an increasing volume of data to be searched, and ii) an increasing number of `candidate' pulsar detections arising from that data, requiring analysis. Whilst almost all are caused by noise or interference, these are often indistinguishable from real pulsar detections. Deciding which candidates should be studied is therefore difficult. Indeed it has become known as the `candidate selection problem'. This thesis presents an interdisciplinary study of the selection problem, with the aim of developing a new method able to mitigate it. Specifically for future pulsar surveys undertaken with the Square kilometre Array (SKA). Through a combination of critical literature evaluations, theoretical modelling exercises, and empirical investigations, the selection problem is described in-depth here for the first time. It is shown to be characterised by the dominance of Gaussian distributed noise signals, a factor that no existing selection method accounts for. It also reveals the presence of a significant trend in survey data rates, which suggest that candidate selection is transitioning from an off-line processing procedure, to an on-line, and real-time, decision making process. In response, a new real-time machine learning based method, the GH-VFDT, is introduced in this thesis. The results presented here show that a significant improvement in selection performance can be achieved using the GH-VFDT, which utilises a learning procedure optimised for data characterised by skewed class distributions. Whilst the principled development of new numerical features that maximise the separation between pulsars and Gaussian noise, have also greatly improved GH-VFDT pulsar recall. It is therefore concluded that the sub-optimal performance of existing selection systems, is due to a combination of poor feature design, insensitivity to noise, and an inability to deal with skewed class distributions.
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Petri, Jérôme. "Structure électromagnétique globale autour des pulsars." Université Louis Pasteur (Strasbourg) (1971-2008), 2002. http://www.theses.fr/2002STR16233.
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Pancrazi, Benoît. "Étude multilongueur d'onde des pulsars milliseconde." Toulouse 3, 2011. http://thesesups.ups-tlse.fr/1525/.
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Les pulsars milliseconde (MSPs) sont des étoiles à neutrons caractérisées par une période de rotation de l'ordre de la milliseconde. Ils sont très probablement accélérés (recyclés) dans des systèmes binaires par transfert de moment cinétique lors d'une phase d'accrétion. A l'image des pulsars normaux, les MSPs sont principalement détectés et observés dans le domaine radio, mais des observations récentes à haute énergie ont montré que certains objets sont également sources de rayons X et gamma. Néanmoins, la question de l'origine de l'émission de haute énergie des MSPs est toujours ouverte. Malgré les avancées théoriques, les grandes dissemblances observées parmi les quelques objets détectés dans les domaines X et gamma ne permettent actuellement pas de répondre à cette question. Cette thèse est consacrée à l'étude des MSPs dans les domaines X, gamma et radio. Grâce à la connaissance des paramètres de rotation déterminés en radio, nous effectuons une recherche systématique de pulsations dans l'émission de haute énergie provenant de six MSPs. La comparaison des profils mesurés dans les différents domaines de longueur d'onde combinée à une étude spectrale fournit des éléments nécessaires à la compréhension de l'origine de l'émission. Ces analyses ont révélé l'existence d'un rayonnement X produit par deux nouveaux objets à ajouter à l'échantillon connu (composé de dix objets avant ce travail), dont l'un est vraisemblablement pulsé, ainsi que de la probable présence de pulsations dans le signal X provenant de trois autres MSPs. Dans le domaine gamma, la détection d'un nouveau signal pulsé vient enrichir l'échantillon actuel de onze MSPs émetteurs gamma. Les résultats suggèrent des origines différentes de l'émission non-thermique dans les domaines X et gamma et soutiennent l'idée que la partie thermique de l'émission X est produite par le réchauffement des calottes polaires. Dans le domaine gamma, la détection de huit amas globulaires (GCs) ouvre de nouvelles perspectives quant à la découverte de nouveaux MSPs. En effet, il est très probable que ces amas denses et âgés d'étoiles liées entre elles par la gravité abritent une large population de MSPs, étant donné le scénario d'accélération de ces derniers. Notre analyse montre que les caractéristiques spectrales mesurées en gamma pour les GCs correspondent à celles attendues pour les MSPs. En suivant l'hypothèse d'une émission intégrée de l'amas due à des MSPs, nous avons pu estimer le nombre de MSPs dans chaque amas détecté ainsi que la population totale dans les GCs de la galaxie. Ce type d'étude représente notamment une opportunité d'étudier le rôle des systèmes binaires, responsables de l'accélération des MSPs, dans la stabilité des GCs. La modélisation de profils d'émission X lorsque celle-ci est d'origine thermique permet de déterminer les paramètres intrinsèques des étoiles à neutrons. Des résultats préliminaires de l'ajustement de tels profils grâce à une méthode de Monte Carlo par Chaînes de Markov sont présentés. Nous étudions la qualité des contraintes obtenues par ajustement des profils d'émission en X avec et sans prise en compte de contraintes sur la géométrie dérivées par la modélisation simultanée des profils gamma et radio pour des objets brillants et dont la masse a été mesurée précisément. A terme, cette méthode devrait aboutir à la contrainte précise de l'équation d'état de la matière dense qui compose les étoiles à neutronsMillisecond pulsars (MSPs) are neutron stars with rotation periods of the order of a few milliseconds. They are most likely accelerated in binary systems through the transfer of angular momentum during an accretion phase. Like normal pulsars, MSPs are mostly detected and observed in the radio domain, but recent observations have shown that some of these objects are also X- and gamma-ray sources. Nonetheless, the question of the origin of high-energy emission produced by MSPs is still open. Despite all theoretical advances, large discrepancies amongst the several detected MSPs in the X- and gamma-ray domains make it impossible, at the current time, to answer this question. This thesis is devoted to the study of MSPs in the X-ray, gamma-ray and radio domains. Based on the knowledge of rotational parameters derived from radio observations, I carried out systematic pulsation searches in the signals measured in X- and gamma-rays coming from six MSPs. The comparison of pulse profiles derived in the different energy domains combined with spectral analyses provides essential information for the understanding of the origin of the emission. These analyses revealed the existence of X-ray emission coming from two new objects that add to the small known sample (ten objects before this work), with one possibly showing pulsed emission, as well as signs of pulsations in the signal emitted by three other MSPs. In the gamma-ray domain, the new detection of pulsed emission from one MSP expands the current sample of eleven gamma-ray emitters. The results suggest different origins for non-thermal radiation in the X- and gamma-ray domains. They also support the idea that the thermal X-ray emission is generated by the heating of the polar caps. In the gamma-ray domain, the detection of eight globular clusters (GCs) provides new opportunities for the discovery of new MSPs. It is very likely that these dense, old and gravitationally bound clusters of stars host a large population of MSPs, given their acceleration scenario. Analysis shows that spectral parameters measured in gamma-rays for GCs match those expected for MSPs. Under the assumption that the integrated emission from the cluster is due to that of MSPs, we were able to estimate the number of MSPs in each detected GC and the global population in the Galactic GCs. This type of study is a good opportunity to study the role of binary systems, that are responsible for the acceleration of MSPs, in the stability of GCs. Modelling of X-ray pulse profiles in the case of thermal emission enables the constraint of intrinsic parameters of neutron stars. Preliminary fits of such profiles using Monte Carlo Markov Chains are presented. I study the quality of constraints derived from fitting X-ray profiles, with and without the constraints on geometry obtained from simultaneous modelling of gamma-ray and radio profiles, in the case of bright objects whose mass has been determined accurately. Ultimately, this method should allow the equation of state of neutron stars' dense matter to be tightly constrained
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Gill, Colin D. "Searching for gravitational waves from pulsars." Thesis, University of Glasgow, 2012. http://theses.gla.ac.uk/3754/.
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The work presented here looks at several aspects of searching for continuous gravitational waves from pulsars, often referred to simply as continuous waves or CWs. This begins with an examination of noise in the current generation of laser interferometer gravitational wave detectors in the region below ~100 Hz. This frequency region is of particular interest with regards to CW detection as two prime sources for a first CW detection, the Crab and Vela pulsars, are expected to emit CWs in this frequency range. The Crab pulsar's frequency lies very close to a strong noise line due to the 60 Hz mains electricity in the LIGO detectors. The types of noise generally present in this region are discussed. Also presented are investigations into the noise features present in the LIGO S6 data and the Virgo VSR2 data using a program called Fscan. A particular noise feature present during VSR2 was discovered with the use of Fscan, which I report on and show how it degrades the sensitivity of searches for CWs from the Vela pulsar using this data. I next present search results for CWs from the Vela pulsar using VSR2 and VSR4 data. Whilst these searches did not find any evidence for gravitational waves being present in the data, they were able to place upper limits on the strength of gravitational wave emission from Vela lower than the upper limit set by the pulsars spin-down, making it only the second pulsar for which this milestone has been achieved. The lowest upper limit derived from these searches confines the spin-down energy lost from Vela due to gravitational waves as just 9% of Vela's total spin-down energy. The data from VSR2 and VSR4 are also examined, analysis of hardware injections in these datasets verify the calibration of the data and the search method. Similar results are also presented for a search for CWs from the Crab pulsar, where data from VSR2, VSR3, VSR4, S5 and S6 are combined to produce an upper limit on the gravitational wave (GW) amplitude lower than has been previously possible, representing 0.5% of the energy lost by the pulsar as seen through its spin-down. The same search method is also applied to analyse data for another 110 known pulsars, with five of these being gamma-ray pulsars that have been timed by the Fermi satellite. GWs from the pulsars timed by Fermi are expected at frequencies below 40 Hz, the LIGO detectors are not calibrated below these frequencies but the Virgo detector is. Hence the data used to search for GWs from these pulsars is the Virgo VSR4 data. The other 105 pulsars were analysed using out of date ephemerides obtained for the LIGO S5 run and the data analysed was from the LIGO S6 run, hence the results obtained for these pulsars are presented as an indication of what results can be expected with updated ephemerides only. For these 110 pulsars the spin-down limits were not able to be beaten, although there are a few pulsars for which this may be able to be achieved with an analysis combining all the possible datasets, in particular J1913+1011. The final part of this thesis reports extensions to the search method used for the analyses previously described. The first way in which this search method is extended is the use of a nested sampling algorithm to perform the parameter estimation stage of the analysis which was previously preformed using a MCMC. The nested sampling code also allows for model selection through the computation of the Bayesian evidence, I present results from characterisation tests of this nested sampling search code that demonstrate the equivalence of its results to those from the MCMC and grid based codes. The other extension to the search method looks at a new CW emission mechanism from a neutron star with a pinned superfluid core that is misaligned from the star's principle axes. This emission model predicts CWs at both the stars spin frequency f and twice its spin frequency 2f, providing an extra data channel with which to perform a search when compared to the triaxial rotator model which only emits at 2f. I present the development of a search for the emission from this new model, tests of the algorithm developed using simulated data, and results from a search using actual data from the VSR4 run for CWs from the Crab pulsar. The testing of the search algorithm shows that the posterior for the model is sufficiently complex to inhibit useful parameter estimation, but that the computation of the Bayesian evidence allows one to distinguish between this model and the triaxial rotator given a low SNR signal in the f data channel.
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Lyutikov, Maxim Blandford Roger D. "Coherent emission mechanisms in radio pulsars /." Diss., Pasadena, Calif. : California Institute of Technology, 1998. http://resolver.caltech.edu/CaltechETD:etd-09232008-092435.
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Taris, François. "Pulsars millisecondes et métrologie du temps." Observatoire de Paris (1667-....), 1996. https://hal.science/tel-02095273.
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Après avoir été définie et réalisée au cours des siècles par les astronomes, la mesure du temps est devenue, en 1964, la responsabilité des physiciens qui, grâce aux horloges atomiques, fabriquent des références de temps extrêmement stables et génèrent l'unité de temps du système international. Cependant, après la découverte en 1982 par Backer des pulsars millisecondes, il apparut que la mesure du temps pourrait redevenir une affaire d'astronomes. La stabilité à long terme de ces pulsars et en particulier de PSR1937+21 semblait en effet équivalente et même meilleure que celle des meilleures horloges atomiques dans le monde. Dans ces conditions, une échelle de temps basée sur la rotation des pulsars millisecondes pouvait-elle remplacer le TAI ? Cette thèse est plus particulièrement consacrée à l'étude des possibilités offertes par les pulsars millisecondes en métrologie du temps. Ce travail s'appuie sur des données d'horloges atomiques ou de références de temps, et sur les datations de signaux pulsars effectuées à l'Observatoire de Paris, auquel est rattaché la station de radioastronomie de Nancay. Il est bien connu, depuis plusieurs années, que l'ajustement des paramètres d'un pulsar absorbe tous les bruits à basses fréquences présent dans les résidus de chronométrage. Les courbes de stabilité, à cause de cet ajustement, sont donc encore probablement biaisées à long terme (quelques années). L'hypothèse la plus communément admise concernant PSR1937+21 est que sa courbe de stabilité continue de montrer un bruit a hautes fréquences même pour les longues périodes d'échantillonnage. Cependant, rien ne s'oppose actuellement au fait que, sur de telles périodes, et lorsque l'ajustement des paramètres n'introduira plus aucun biais dans le calcul de la stabilité, celle-ci exhibe plutôt un bruit a basses fréquences. C'est l'hypothèse de travail qui est retenue ici. Si elle s'avère exacte, et en admettant que la stabilité de PSR1937+21 soit représentative de celle de l'ensemble des pulsars millisecondes, la métrologie du temps ne pourrait plus être considérée comme un des objectifs scientifiques du chronométrage des pulsars. Cette thèse tente également de montrer que les incertitudes actuelles des paramètres de PSR1937+21 sont des incertitudes de types A et qu'aucune détermination des incertitudes de type B, qui conduiraient à la détermination de l'exactitude des paramètres, n'a jamais été effectuée.
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張益軍 and Yijun Zhang. "Pulsar statistics in our galaxy." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2000. http://hub.hku.hk/bib/B31225585.
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Bondonneau, Louis. "Première caractérisation de la population de pulsars radio à basses fréquences avec NenuFAR." Thesis, Orléans, 2019. http://www.theses.fr/2019ORLE3032.
Full textAbstract:
Un pulsar est une étoile à neutrons en rotation rapide (typiquement un tour par seconde) dont le faisceau d’émission radio provenant des pôles magnétiques de l'étoile balaie l’univers. Quand le faisceau intercepte la Terre, le pulsar est détectable comme une série de pulsations régulières dans dans un vaste domaine de longueurs d'onde, de la radio jusqu'aux rayons gamma. Au cours de cette thèse j’ai utilisé, adapté et développé des méthodes d'analyses ainsi qu'un pipeline de traitement de signal en temps réel afin d’étudier les signaux radio des pulsars dans la gamme des basses fréquences utilisée par les radiotélescopes LOFAR(LOw Frequency Array) et NenuFAR (New Extension in Nançay Upgrading loFAR).NenuFAR est le nouvel instrument de la station de Radioastronomie de Nançay, construit pour observer le ciel entre 10 et 85 MHz de fréquence. C'est un réseau compact de nouvelle génération, constitué à terme de 1938 antennes phasées analogiquement et numériquement. Depuis le début de ma thèse, j'ai participé activement au développement, puis à la recette de ce nouvel instrument, jusqu'à son ouverture à la communauté scientifique le 1er juillet 2019 dans le cadre d’un appel à "Early Science".Le premier chapitre de ma thèse est consacré à la description du phénomène « pulsar », de l'émission du rayonnement dans la magnétosphère de l'étoile à neutrons jusqu'à l'observation du signal par le radiotélescope. Le second chapitre décrit l’instrumentation des radiotélescopes utilisés pendant la thèse, et en particulier, la conception du dédisperseur « pulsar » cohérent en temps réel de NenuFAR (LUPPI) et du « pipeline » de traitement des observations. Le troisième chapitre est réservé à l’étude d’une centaine de pulsars observés à basse fréquence avec LOFAR. Les données sont issues de deux relevés, l’un effectué avec le coeur de LOFAR (situé aux Pays-Bas) et l’autre avec la station LOFAR de Nançay. Finalement, le dernier chapitre est dédié à la mise en service de NenuFAR et aux premiers résultats scientifiques obtenus pour les observations des pulsars. Ce chapitre décrit en particulier les tests pour les différents modes d'observation (dédispersion cohérente, multi-beam, single pulse, enregistrement en forme d'onde) et le résultat du premier grand relevé du ciel Nord par NenuFAR, qui a permis à partir de l’observation de 500 pulsars la détection de 130 sources, dont plus de 50 pour la première fois à ces fréquences. J'y présente également le programme scientifique du projet clé pulsars de NenuFAR, que j'ai fortement contribué à mettre en placeA pulsar is a rapidly rotating neutron star (typically one revolution per second). As it rotates, the beamed radio emission from the star's magnetic poles scans the universe. When the beam intercepts the Earth, the pulsar is detectable as a series of regular pulses over a wide wavelength range, from radio to gamma rays. During this thesis I used, adapted and developed analysis methods and a real-time signal processing pipeline to study pulsar radio signals in the low frequency range used by the radio telescopes LOFAR (LOw Frequency Array) and NenuFAR (New Extension in Nançay Upgrading loFAR).NenuFAR is the new instrument of the Nançay Radio Astronomy Station, built to observe the sky at frequency between 10 and 85 MHz. It is a compact array of the last generation of radio telescopes. Ultimately it will be composed of 1938 antennas with analog and digital phasing. Since the beginning of my thesis, I have been actively involved in the development of this new instrument, until it was opened to the scientific community on July 1, 2019 as part of a call for "Early Science".The first chapter of my thesis is devoted to the description of the "pulsar" phenomenon, from the emission of radiation in the magnetosphere of the neutron star to the observation of the signal by a radio telescope. The second chapter describes the instrumentation of the radio telescopes used during the thesis, and in particular the design of the NenuFAR coherent real-time "pulsar" dedispersion system (LUPPI) and the pipeline for the processing of the observations. The third chapter presents the study of about 100 pulsars observed at low frequency with LOFAR. The data are based on two surveys, one with the LOFAR core (located in the Netherlands) and the other with the LOFAR station in Nançay. Finally, the last chapter is dedicated to the commissioning of NenuFAR and the first scientific results obtained from pulsars observations. This chapter describes in particular the tests for the different observation modes (coherent dedispersion, multi-beam, single pulse, waveform recording) and the result of the first major survey of the North sky by NenuFAR. Based on the observation of 500 pulsars, this survey allowed the detection of 130 sources, including more than 50 which had never been detected at these frequencies before. I also present the scientific program of the NenuFAR pulsar Key Project, to which I have strongly contributed
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Zhang, Li, and 張力. "High energy radiation from rotation-powered pulsars." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1998. http://hub.hku.hk/bib/B30088173.
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Zhang, Li. "High energy radiation from rotation-powered pulsars /." Hong Kong : University of Hong Kong, 1998. http://sunzi.lib.hku.hk/hkuto/record.jsp?B19737609.
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Venter, Christo. "Millisecond pulsars and pulsar wind nebulae as sources of gamma rays and cosmic rays / C. Venter." Thesis, North-West University, 2008. http://hdl.handle.net/10394/2067.
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Lundgren, Scott Campbell. "A multi-wavelength study of rotation-driven pulsars." Arecibo, Puerto Rico ; Ithaca, N.Y. : National Astronomy and Ionospheric Center, Arecibo Observatory, 1994. http://catalog.hathitrust.org/api/volumes/oclc/29573307.html.
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Jia, Jianjun. "The phase-resolved spectra of the crab pulsar." Click to view the E-thesis via HKUTO, 2005. http://sunzi.lib.hku.hk/hkuto/record/B36379918.
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Edwards, Russell T., and Russell Edwards@csiro au. "Pulsar searching." Swinburne University of Technology, 2001. http://adt.lib.swin.edu.au./public/adt-VSWT20050323.141044.
Full textAbstract:
This thesis reports the results of two pulsar survey projects conducted at the Parkes 64-m radio telescope in New South Wales, Australia.
The first, the Swinburne Intermediate Latitude Pulsar Survey, covered a large region of the southern Galaxy flanking that of the ongoing Galactic plane survey. We used the 13-feed 20 cm
'multibeam' receiver package to achieve this broad sky coverage in a short observing campaign with 14 days� total integration time. The survey proved remarkably successful, detecting 170 pulsars, 69 of which were new discoveries. Eight of the new discoveries possess small periods and period derivatives indicative of 'recycling', an hypothesis supported by the fact that six of them are in circular orbits with probable white dwarf companions. Pulse timing measurements have revealed that two of the white dwarfs are massive CO or ONeMg dwarfs. The mass of one of them (the companion to PSR J1157�5112) exceeds 1.14 M, providing the most convincing evidence to date for the production of 'ultra-massive' ONeMg white dwarfs as the end result of stellar evolution on the asymptotic giant branch (albeit with mass transfer indicated). PSR J1757�5322 also possesses a heavy white dwarf companion, in a close 11-h orbit. The proximity of the massive companion leads to significant relativistic orbital evolution and the effects of this will be measurable by pulsar timing in the coming decades. Under general relativity, the gravitational wave power radiated from the system is sufficient to cause coalescence in an event which will have dramatic and unknown consequences. Such events are possible gamma-ray burst sources, and the remnants could include isolated millisecond pulsars, close eclipsing binaries or pulsar planetary systems. The remaining four pulsar binaries show some discrepancies with the bulk of previously known low mass binary pulsars (LMBPs). PSR J1618-39 is
in a 23-d orbit, filling what previously appeared to be a gap in the orbital period distribution. PSR J1745-0952 has a relatively long pulse period (19 ms) and along with PSR J1618-39 (12 ms) may have experienced a different evolutionary history
to the majority of previously known LMBPs. A ninth pulsar discovered in the survey may also be recycled. The mean pulse profile of PSR J1411�7404 is exceedingly narrow and lies in stark contrast to that of other pulsars of similar pulse period. In the past the only other pulsars
known with anomalously narrow profiles were believed (for other reasons) to have been recycled, and this fact in combination with the low period derivative measured in timing analysis of PSR J1411�7404 leads me to suggest that it, too, may have been recycled. If this is the case, it is possible that the recycling took place in a
system similar in configuration to the progenitors of the double neutron star systems,
but that sudden mass loss or an unfavourably oriented kick in the birth event of the second neutron star disrupted the system, leaving an isolated, mildly recycled pulsar. The second pulsar survey program conducted for this work was a targeted search of southern globular clusters. We used a baseband recording system to provide unprecedented time resolution (typically 25 �s). The large number of channels and short sampling interval achievable in software filterbanks, in combination with the ability to coherently remove most of the interstellar dispersion from clusters with previously known pulsars, made us the first to achieve a relatively flat sensitivity response to pulsars of ~10−3.5�10 s. This characteristic is vital if we are to constrain the true period distribution of millisecond pulsars, an important task in the evaluation of alternative equations of state for nuclear matter. We detected six millisecond pulsars and produced pulse profiles of higher resolution than were previously available. The basic sensitivity of the search was not high enough to detect any new pulsars, however the work demonstrates that the approach is feasible with the use of currently available high-performance computing resources (such as the Swinburne workstation cluster), and is capable of delivering excellent sensitivity characteristics.
It is expected that future searches of this kind, of which this is the first, will achieve the goal of sampling the true pulse period distribution within a few years.
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Hotan, Aidan Wade, and n/a. "High-precision observations of relativistic binary and millisecond pulsars." Swinburne University of Technology, 2006. http://adt.lib.swin.edu.au./public/adt-VSWT20060822.151026.
Full textAbstract:
The technique of pulsar timing reveals a wealth of new information when a precision
of ∼1 μs or better is reached, but such precision is difficult to achieve. This
thesis describes a series of very high precision timing observations that improve our
knowledge of the targeted pulsar systems. We begin by describing a newly-developed
baseband recording and coherent dedispersion system (CPSR2), along with a new
object-oriented software development environment for pulsar data processing. Data
obtained with this new instrument during a 3 year observing campaign at the Parkes
64m radio telescope are analysed in a number of novel ways.
The mean profile of PSR J1022+1001 is shown to be stable on timescales of a few
minutes, in contrast with previously published claims. We obtain a level of precision
an order of magnitude better than any previous timing of this pulsar. In addition,
we observe dramatic changes in the mean profile of the relativistic binary pulsar
J1141?6545, which broadens by ∼50% over the time span of our observations. This
is interpreted as evidence for secular evolution of the line of sight to the emission
cone, caused by General relativistic geodetic precession which tilts the spin axis of
the pulsar. High precision CPSR2 observations of the extraordinary double pulsar
binary system are presented and we construct calibrated, mean polarimetric profiles
for PSR J0737?3039A, in two frequency bands. These profiles provide a reference
against which future profile evolution may be detected, given that we expect geodetic
precession to alter the observed mean profile on an even shorter time scale than for
PSR J1141?6545.
The bulk of this thesis involves timing a selection of millisecond pulsars whose
physical characteristics should allow the highest precision to be obtained. We measure
several new proper motions and parallax distances. Shapiro delay is used to
constrain the inclination angles and component masses of several of the binary systems
in our source list. In addition, subtle periodic variations of the orbital parameters
of two nearby binary millisecond pulsars are detected and attributed to annual
orbital parallax, providing additional constraints on their three-dimensional orbital
geometries. Future observations of these two sources may lead to more stringent
tests of post-Keplerian gravitational theories. Finally, we use the timing residuals
of one very stable source (PSR J1909?3744) as a reference against which we time
PSR J1713+0747 with a root-mean-square precision of 133 ns, amongst the best
timing residuals ever obtained. This result is an important step in the search for
long-period gravitational waves using pulsar timing arrays.
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Hotan, Aidan Wade. "High-precision observations of relativistic binary and millisecond pulsars." Australasian Digital Theses Program, 2006. http://adt.lib.swin.edu.au/public/adt-VSWT20060822.151026.
Full textAbstract:
Thesis (PhD) - Swinburne University of Technology, 2006.A dissertation presented in fulfillment of the requirements for the degree of Doctor of Philosophy, Swinburne University of Technology - 2006. Typescript. Includes bibliographical references (p. 182-190).
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Zhang, Shimin. "Model of high energy emission from young pulsars." View the Table of Contents & Abstract, 2008. http://sunzi.lib.hku.hk/hkuto/record/B39630882.
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Li, Yi, and 李毅. "The thermal afterglow of a pulsar glitch in non-spherical symmetric case." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1998. http://hub.hku.hk/bib/B31215580.
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Zhang, Shimin, and 張世民. "Model of high energy emission from young pulsars." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2008. http://hub.hku.hk/bib/B39707453.
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Li, Yi. "The thermal afterglow of a pulsar glitch in non-spherical symmetric case /." Hong Kong : University of Hong Kong, 1998. http://sunzi.lib.hku.hk/hkuto/record.jsp?B19737051.
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Keane, Evan. "The transient radio sky." Thesis, University of Manchester, 2010. https://www.research.manchester.ac.uk/portal/en/theses/the-transient-radio-sky(37c08735-cd96-4598-a8b9-2d24ef9e871d).html.
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The high time-resolution radio sky represents unexplored astronomical territory where the discovery potential is high. In this thesis I have studied the transient radio sky, focusing on millisecond scales. As such, this work is concerned primarily with neutron stars, the most populous member of the radio transient parameter space. In particular, I have studied the well known radio pulsars and the recently identified group of neutron stars which show erratic radio emission, known as RRATs, which show radio bursts every few minutes to every few hours. When RRATs burst onto the scene in 2006, it was thought that they represented a previously unknown, distinct class of sporadically emitting sources. The difficulty in their identification implies a large underlying population, perhaps larger than the radio pulsars. The first question investigated in this thesis was whether the large projected population of RRATs posed a problem, i.e. could the observed supernova rate account for so many sources. In addition to pulsars and RRATs, the various other known neutron star manifestations were considered, leading to the conclusion that distinct populations would result in a 'birthrate problem'. Evolution between the classes could solve this problem - the RRATs are not a distinct population of neutron stars. Alternatively, perhaps the large projected population of RRATs is an overestimate. To obtain an improved estimate, the best approach is to find more sources. The Parkes Multi-beam Pulsar Survey, wherein the RRATs were initially identified, offered an opportunity to do just this. About half of the RRATs showing bursts during the survey were thought to have been missed, due to the deleterious effects of impulsive terrestrial interference signals. To remove these unwanted signals, so that we could identify the previously shrouded RRATs, we developed new interference mitigation software and processing techniques. Having done this, the survey was completely re-processed, resulting in the discovery of 19 new sources. Of these, 12 have been re-detected on multiple occasions, whereas the others have not been seen to re-emit since the initial discovery observations, and may be very low burst-rate RRATs, or, isolated burst events. These discoveries suggest that the initial population estimate was not over-estimated - RRATs, though not a distinct population, are indeed numerous. In addition to finding new sources, characterisation of their properties is vital. To this end, a campaign of regular radio observations of the newly discovered sources, was mounted, at the Parkes Observatory, in Australia. In addition, some of the initially identified RRATs were observed with the Lovell Telescope at Jodrell Bank. These have revealed glitches in J1819-1458, with anomalous post-glitch recovery of the spin-down rate. If such glitches were common, it would imply that the source was once a magnetar, neutron stars with the strongest known magnetic fields of up to 10¹⁵ gauss. The observations have also been used to perform 'timing' observations of RRATs, i.e. determination of their spin-down characteristics. At the beginning of this thesis, 3 of the original sources had 'timing solutions' determined. This has since risen to 7, and furthermore, 7 of the newly discovered sources now also have timing solutions. With this knowledge, we can see where RRATs lie in period-period derivative space. The Parkes RRATs seem to be roughly classifiable into three groupings, with high observed nulling fractions - normal pulsars, high magnetic field pulsars and old, 'dying' pulsars. It seems that RRATs and pulsars are one and the same. When a pulsar is more easily detected in searches for single bright pulses, as opposed to in periodicity searches, we label it a RRAT. Such searches impart a selection effect on the parameter space of possible sources, in both nulling fraction and rotation period. In this sense, an observational setup could be designed to make any pulsar appear as a RRAT. For realistic survey parameters however, this is not the case, and the groups mentioned above seem to be the most likely to appear as RRATs. In fact, we can utilise RRAT searches to identify neutron stars, difficult to find by other means, in particular high-magnetic field pulsars, and pulsars approaching the pulsar "death valley". Some of the RRATs are well explained as being distant/weak pulsars with a high modulation index, others seem to be nulling pulsars. This highlights the incomplete knowledge of nulling behaviour in the pulsar population. It seems that there may be a continuum of nulling durations, under a number of guises, from 'nulling pulsars' to 'RRATs' to 'intermittent pulsars'. In fact this nulling may fit into the emerging picture, whereby pulsar magnetospheres switch between stable configurations.
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RIGOSELLI, MICHELA. "X-ray emission from the magnetic polar caps of old rotation-powered pulsars." Doctoral thesis, Università degli Studi di Milano-Bicocca, 2020. http://hdl.handle.net/10281/277373.
Full textAbstract:
Le stelle di neutroni sono il risultato dell'evoluzione di stelle massive dopo l'esplosione di supernova. Il progetto di questa Tesi di PhD consiste nello studio dell'emissione di raggi X da parte di stelle di neutroni isolate di età superiore a 100000 anni. Sono stati analizzati dati provenienti dal satellite XMM-Newton (ESA). Per estrarre la miglior informazione possibile dai dati, ho implementato un metodo di maximum likelihood (ML) e l'ho utilizzato per estrarre spettri e profili pulsati di pulsar vecchie in banda X, che poi sono stati analizzati con raffinati di emissione.
La Tesi è strutturata come segue: nei primi tre capitoli illustro le proprietà principali delle stelle di neutroni, con particolare attenzione ai processi termici e non termici che producono raggi X. I raggi X non termici sono prodotti da particelle relativistiche accelerate da campi elettromagnetici; una frazione di queste particelle viene accelerata verso la superficie della stella, e riscalda le zone delle calotte polari magnetiche. La componente termica, che può essere prodotta dall'intera superficie o da una parte, viene solitamente descritta come un corpo nero; tuttavia, la presenza di intensi campi magnetici superficiali influenza le proprietà della materia, e la radiazione emessa è ampiamente anisotropa.
Nel Capitolo 4 descrivo come ho generato spettri e profili pulsati sintetici, utilizzando modelli di emissione che considerano calotte polari ricoperte di un'atmosfera di idrogeno magnetizzata. Mi sono basata su un software esistente che, dato un certo set di parametri relativi alle proprietà fisiche della stella, stima l'intensità della radiazione prodotta. Successivamente, esso somma i contributi degli elementi di superficie che sono visibili all'osservatore alle differenti fasi di rotazione. Quindi, nel Capitolo 5 descrivo come ho implementato un software di analisi che si basa sul metodo di ML. Dato un certo modello, esso stima i parametri più probabili che ricostruiscono i dati osservati, nella fattispecie il numero di conteggi relativo alla sorgente e al background. Ho validato il metodo e dimostrato che esso è particolarmente efficace per sorgenti deboli, quali sono la maggior parte delle pulsar vecchie.
Successivamente, ho applicato i metodi finora descritti ad alcune pulsar vecchie. Nel Capitolo 6 riporto l'analisi di PSR J0726-2612, una pulsar radio che ha alcune delle caratteristiche delle XDINSs: un periodo di rotazione lungo, un intenso campo magnetico, ed emissione X termica. Grazie ad un'analisi congiunta dello spettro e del profilo pulsato, ho mostrato che la presenza di impulsi radio in PSR J0726-2612, così come la loro assenza nelle XDINSs, potrebbe essere dovuta ad un'orientazione differente rispetto all'osservatore. Nel Capitolo 7 presento il caso di PSR B0943+10, una pulsar con emissione X sia termica sia non termica ma che, nonostante sia un rotatore allineato, ha una grande frazione pulsata. Sono riuscita a riappacificare i due diversi aspetti grazie ad un'attenta analisi di ML e all'intrinseca anisotropia dell'emissione termica prodotta da un'atmosfera magnetizzata. Nel Capitolo 8 ho invece applicato il metodo di ML su sette pulsar poco brillanti, delle quali quattro avevano diverse analisi già pubblicate in letteratura, ma con risultati discordanti, mentre le altre tre non erano ancora state rivelate in banda X. Ho trovato tracce evidenti di emissione termica solo in due oggetti, più un accenno nello spettro pulsato di un terzo oggetto.
Infine, ho considerato tutte le pulsar vecchie che emettono raggi X termici e ho confrontato le misure di temperatura, raggio e luminosità alle aspettative dei modelli teorici. In particolare, ho scoperto che le aree di emissione sono generalmente in accordo con quelle previste dal modello di dipolo magnetico rotante, purché vengano considerati anche effetti di proiezione geometrica e modelli di emissione realistici, quali appunto i modelli di atmosfera magnetizzata.Neutron stars are the remnants of massive stars whose cores collapse during the supernova explosions. The project of this PhD Thesis consisted in the study of the X-ray emission from isolated neutron stars older than about 100000 years. The work was based mainly on data obtained with the XMM-Newton satellite (ESA). To extract the best possible information from the data, I implemented a maximum likelihood (ML) technique and used it to derive the X-ray spectra and pulse profiles of several old pulsars, that were then studied with state-of-the-art models of X-ray emission. The Thesis is structured as follows: in the first three chapters I outline the main properties of neutron stars, with a major focus on the thermal and nonthermal processes that produce X-rays. The nonthermal X-rays are produced by relativistic particles accelerated by rotation-induced electric fields and moving along the magnetic field lines. A fraction of these particles is accelerated backward and returns on the stellar surface, heating the magnetic polar caps. The thermal component, that can be produced by the whole stellar surface or by small hot spots, can be described, in a first approximation, by a blackbody. However, the presence of intense surface magnetic fields strongly affects the properties of matter, and the emergent radiation is widely anisotropic. In Chapter 4, I describe how I generated synthetic spectra and pulse profiles using thermal emission models that consider polar caps covered by a magnetized hydrogen atmosphere or with a condensed iron surface. I relied on an existing software that, given a set of stellar parameters, evaluates the emerging intensity of the radiation. A second software, which I adapted on the sources I analyzed in the Thesis, collects the contribution of surface elements which are in view at different rotation phases from a stationary observer. Then, in Chapter 5, I describe how I implemented an analysis software that relies on the ML method. It estimates the most probable number of source and background counts by comparing the spatial distribution of the observed counts with the expected distribution for a point source plus an uniform background. I demonstrated that the ML method is particularly effective for dim sources, as most old pulsars are. Subsequently, I applied the methods described above to some old pulsars. In Chapter 6, I report the analysis of PSR J0726-2612, a radio pulsars that shares some properties with the radio-silent XDINSs, as the long period, the high magnetic field, and the thermal X-ray emission from the cooling surface. Thanks to an in-depth analysis of the combined spectrum and pulse profile, I showed that the presence of radio pulses from PSR J0726-2612, as well as the absence from the XDINSs, might simply be due to different viewing geometries. In Chapter 7, I present the case of PSR B0943+10, a pulsar with a nonthermal and thermal X-ray spectrum but that, despite being an aligned rotator, has a large pulsed fraction. I could reconcile the two opposite properties analyzing with the ML the spectrum and the pulse profile, and considering the magnetic beaming of a magnetized atmosphere model, that well fits the thermal component. In Chapter 8, I applied the ML method to seven old and dim pulsars, of which four had controversial published results, and three were so far undetected. I found convincing evidence of thermal emission only in the phase-averaged spectrum of two of them, plus a hint for a thermal pulsed spectrum in a third object. Finally, I considered all the old thermal emitters and I compared their observed temperatures, radii and luminosities to the expectations of the current theoretical models for these objects. In particular, I found that the emitting area are generally in agreement with the polar cap regions evaluated in a dipole approximation, if the combined effects of geometry projections plus realistic thermal models (as the magnetic atmosphere) are taken into account.